1. Technical Field
The present invention relates to fuel valves, and in particular, to highly accurate valves suitable for use in regulating fuel to the burners of turbine engine systems.
2. Description of the Related Art
Modern gas turbines used in the generation of power typically employ various lean fuel/air mixing techniques, largely to reduce the levels of harmful nitrogen oxides exhausted. Altering the fuel to air ratio to reduce NOX emissions can cause “screech” or instability in the combustion such that the burner flame is inconsistent or unsustainable. The unstable burning generates pressure fluctuations in the combustion cans. These pressure fluctuations cause oscillating waves in the combustion can on the order of several hundred cycles per second. This high frequency vibration can cause rapid breakdown of the combustion components which can send particles or other debris to the turbine blades and thereby cause severe structural damage to the turbine.
Sophisticated combustion control systems have been developed to continuously monitor and actively stabilize the combustion of modern gas turbines to avoid or minimize these adverse affects on the turbine. These systems usually include high speed pressure transducers located, for example, in the combustion can to sense the pressure oscillations arising from the unstable burning. The transducers provide pressure signals to a control computer which processes the signals according to various algorithms to control various combustion components or parameters to counteract the pressure oscillations. Typically, this involves pulsing the fuel sent to the combustion cans at very high rates commensurate with the frequency of the pressure oscillations. This is ordinarily accomplished by rapidly operating the fuel metering valves. However, many metering valves are insufficiently responsive and accurate to the detriment of the turbine performance.
Modern gas turbine engines used for power generation are very large and capable of a continuous power output between 200–500 megawatts. Such high output requires significant fuel consumption on the order of 200–400 gallons per minute. To effectively burn this high flow volume, typical industrial gas turbine divide the fuel flow and burning into several combustion cans, often more than 10. The combustion cans are typically arranged in an array, such that burned fuel in each combustion can provides a flame front that effects a pressure change that drives the turbine blades. The pressure variation is dependent upon the temperature of the flame front. The higher the flame temperature, the greater the change in pressure, and thus the more power output from the turbine. However, the overall flame temperature is actually an average of the flame temperature at each burner or combustion can. The temperature gradient profile of the several burners is defined by its “pattern factor”, which is typically defined as the difference between the peak and average combustor exit temperatures divided by the average exit temperature expressed as:
      Pattern    ⁢                  ⁢    Factor    =                              T          ⁡                      (            exit            )                          peak            -                        T          ⁡                      (            exit            )                          avg                            T        ⁡                  (          exit          )                    avg      
Ideally, the average flame temperature of all combustion cans should equal the flame temperature at the flame front so that the pattern factor is zero. However, the average temperature is actually some valve less than the peak temperature, resulting in a positive pattern factor value. Should one or more combustion cans have a significantly low temperature, the average flame front temperature can vary significantly from the peak temperature, thereby resulting in a high pattern factor, and inefficient operation of the turbine.
Systems for controlling the turbine pattern factor are known. Typically, such systems include an electronic control that uses temperature feedback signals at each combustor can to regulate flow to the burners. Individual control fuel control valves are used so that fuel flow to selected burners can be controlled. As with the previously described problem, these valves are often insufficiently responsive and inaccurate, particularly at the high flow rate and pressure experienced in such applications, to achieve the desired control of the burner temperature profile. Moreover, these valves are also very susceptible to deterioration due to the contaminated and aggravated temperature environments in which they are used, such that the accuracy problem becomes exacerbated with extended use.
Accordingly, an improved fuel valve is needed for use in a pattern factor control system for a gas turbine.